Heat shield for incandescent lamp



N0v.25,1958 @.HBIGGS Em. 2,862,125

HEAT SHIELD FOR INCANDESCENT LAMP Filed Jan; 6, 1956 2,862,125 v 'HEAT SHIELD Fon INCANDESCENT LAMP Orrick H. Biggs, Beverly, and Robert F. Reed, Salem, Mass., assignors to Sylvania Electric Products Inc., Salem, Mass., a corporation of Massachusetts Application January 6, 1956, Serial No. 557,692

6 Claims. (Cl. 313-33) This invention relates to incandescent lamps and more particularly to lamps for use in projectors.

Projection lamps are operated at considerably higher bulb and filament temperatures than ordinary incandescent lamps, of comparable envelope size, employed for general lighting purposes. Accordingly, projection lamps have a much shorter life than ordinary lamps because of a high rate of filament evaporation. In order to limit` filament evaporation, such lamps are normally filled with an inert gas such as nitrogen, argon, or a combination of both, to a pressure somewhat below atmospheric, at room temperature. Since the internal pressure of the lamp increases in operation to al level considerably above atmosphere, the envelope is subjected to forces tending to deform the envelope outwardly from within. In some cases, the lamp may be filled to a pressure above atmosphere even at room temperature.

When a projection lamp, having a tubular envelope or bulb closed at one end and bearing a contact base at the other, is operated in base-down position, the envelope generally remains sufficiently rigid to be unaffected by such internal pressure. In this position, there is an adequate distancebetween the filament and the upper end of the envelope to allow convection currents of gas heated by the filament to cool before contacting the envelope. Additionally, forced air cooling of the lamp is greatly facilitated when the lamp is positioned vertically.

However, recent developments in projector design require a lamp which is operable in a horizontal position, that is, with the envelope extending laterally from the base at one side. When a lamp is'thus mounted, the vertical distance between the filament and the envelope is much less than when the same lamp is positioned vertically. The heated gas rising from the filament carries sufficient heat to render the envelope plastic in the region above the filament. Moreover, this upper region will receive only a minimum cooling effect from any cooling air which is forced to flow laterally over the outside of the envelope. As a result, the envelope, rendered plastic by heat from the filament and under the pressure of the gas till, bulges outwardly in its upper region. This deformation of the envelope renders the lamp useless after it has been employed for a period of time equal to only a fraction of its normal life. Furthermore, such deformation of the lamp envelope may seriously injure the projector in which the lamp is used. Y

It is therefore an object of our invention to provide means for protecting the envelope of a high temperature incandescent lamp from heat produced by the filament thereof.

Various means of accomplishing this have been suggested. One of these is the use of a hard glass envelope. Such a change in material would, however, entail many manufacturing problems and result in a corresponding increase in cost. Moreover, it would not of itself solve the problem. Another alternative expedient is an increase in the diameter of the envelope to increase the '2,862,125 Patented Nov. 25, 1958 vertical distance between the filament and the envelope. Such an increase is objectionable because the lamp is designed for use with a reflector and lens which would then have to be placed at a greater distance from the filament, with consequent reduction in the efficiency of light utilization of the combination.

We have discovered, however, that the overheating and bulging of the glass can be prevented by providing an internal, heat shield between the filament and the upper portion of the envelope. Such a shield can serve to deflect the convection currents away from the overheated portion of the bulb, that is, from the top of the bulb when the latter is in its operating position. We have also discovered, that when solid shields, such as shields of metal foil are used, the effect is merely to transfer the bulges from the upper portion to the sides of the envelope. Since bulges at the sides are even more likely to damage the projectors reflector and lens than would a similar bulge above the filament, such solid shields are accordingly unsatisfactory.

We have further discovered lthat the overheating at the top of the bulb can be prevented, without overheating the sides, by making the shield foraminous. The use of a shield of that type reduces the convection currents to the top of the bulb,without at the same time, merely ldeflecting them to the sides. In other words, the shield breaks up or disperses the convection currents to distribute the heat over a greater portion of the bulb. To facilitate the dispersion, it will generally be desirable to space the edge of the shield from the envelope wall.

Such shield may be formed of woven screen, of lattices, of strips welded together, of pieces of foil having a multitude of perforations, or of other suitable materials. It has been found that when a relatively closed structure is employed, the envelope bulges at the sides. On the other hand, the use of too open a structure resulted in the formation of a bulge above the filament.

Since the convection currents will be greatest immediately above the filament, the shielding effect can be made greatest in that region and smaller in the region spaced out from the verti-cal plane of the filament. This can be done, for example, by having the ratio of closed to open area greatest at the middle of the shield, where it is attached to a filament support, and smallest near the longitudinal Iedges of the shield.

An additional object of our invention is to provide for projection lamps a foraminous internal shield which is easy and cheap to manufacture and rugged in use.

Further objects, advantages, and novel features of our invention will be clarified from the following description of a preferred embodiment thereof, taken in connection with the accompanying drawings in which:

Figure l is a view in front elevation of a projection lamp embodying a heat shield according to our invention;

Figure 2 is a view of the projection lamp in cross section taken along the line 2 2 of Figure 1;

Figure 3 is a detail drawing of the shield before bending; and

Figure 4 is an end view of the shield after bending.

Referring now to Figure l, a projection lamp employing a heat shield according to our invention will now be more particularly described. The lamp comprises a coiled filament 12 enclosed within a glass envelope or bulb 14. The filament 12 is arranged between support wires 16 and 18 in a plane normal to the direction of maximum light emission and is in electrical communication with contact pins 20 through a coil insert 22 at each end of the filament. The filament 12 is positioned on wires 24 extending from a glass bridge 26 at each end. Each bridge 26 is in turn insulatedly fixed upon the support wires 16, 1S. The envelope 14 is filled 'with nitrogen 3 to a pressure, at room temperature, of 685 mm. of mercury.

A shield indicated at28 is mounted on the support wire I6 which is above the filament 12 when the lamp is vinstalled in a horiontal position. In a 500 watt lamp provided with a 'soft lime glass envelope having a softening point of 696 C., a diameter of 11/2 inches, and a length of approximately 31/2 inches, we employ a shield of nickel foil .G10 inch thick having a length of .95 inch and a width of .75 inch. Openings are formed in the shield and the latter is folded on its longitudinal center line to form two panels `120" apart. The openings consist of a row of four rectangular holes 3l), each .1875 by .145 inch, adjacentieach longitudinal edge and a cen tral row of rectangular holes 32, each .300 inch by .l2i5 inch. The openings are arranged so that adjacent holes are separated by strips .O40 inch wide and a .040 inch border is also provided along each edge. The shield is welded along its fold to the support wire i6, each panel forming an angle of approximately 60 with the plane of the filament. It will be noted that such placement'of the shield 28 on the support wire 'i6 which lies about 7/16 inch from the axis of the bulb leaves Va space of approximately Mi inch between each longitudinal edge of the shield 28 andthe envelope I4.

While the above described shield provides deflection of an adequate portion of convection currents within the envelope to keep the glass above the filament from turning plastic, it is but one of several types which have been tried. Generally, it has been found that screens which were too open in construction allowed the passage of a sufficient amount of the convection currents to render the envelope plastic above the filament. On the other hand, too close a construction deflected a sufficient amount of hot gas to the sides of the envelope that the glass became plastic in that region.

For example, a foil shield l inch long and .70 inch wide and having a series of four 16 inch holes adjacent each longitudinal edge was tried in a 50() watt lamp having the type of envelope described above. This -construction lacked adequate porosity and resulted in objectionable blistering of the envelope at the sides ofthe shield when the lamp was tested. However, when subsequent samples were tested in which a central row of 3 similar holes had been added to the shield, no blistering occurred either at the sides of the shield or above the filament.

Asa further example, a shield of interwoven wires hav ing a diameter of about .G96 inch lwas `built into additional samples. The 'shi'eld'had overall dimensions of /16 inch vvvbyyl'inch and included about l2 longitudinal wires woven `with about 2() transverse wires. Samples havingsuch shields Weretested with resulting blisters at the sides-ofthe shields. However, in subsequent samples porosity of the shield was increased by removing four longitudinal wires'and six transverse Wires and satisfactory results thereby obtained.

From the foregoing'description it will be appreciated that a multiplicityof variations -within the scope of our invention will nowvbe apparent'to'those of ordinary skill in the art. It is therefore not intended to limit the invention to any particular form of shieldbut rather to rely upon the definitions provided `by the appended claims.

What we claim is:

l. In an incandescent lamp having a tubular gas-filled glassrenvelope, support elementsextending longitudinally within said envelope and a filament mounted between said elements, a heat shield comprising: a foraminous member having a plurality of openings and extending longitudinally in said envelope at the side of said filament, said member presenting an obstruction to passage of a portion of the rising gas bearing heat emanating from said filament and allowing another portion of said gas to pass through said openings whereby local overheating of said envelope is avoided when the lamp is operated horizontally with the shield above the filament.

2. In a projector, an incandescent lamp having a horizontally oriented, gas-filled, tubular envelope, an upper and a lower support element extending laterally within said envelope, and a filament mounted in a plane between said elements, a heat shield transverse to the plane of said filament and comprising: a foraminous member, having a plurality of openings, mounted on said upper element above said ,filament and in the path of convection gas currents from said `filament said openings forming passages for a portion of said currents and said member obstructing the remainder of said currents whereby heat from the filament is distributed over a greater portion of the envelopes surface.

3. In a projection lamp having a gas-filled, tubular envelope, two support elements extending longitudinally within said envelope, and a filament mounted in a plane between said elements, a heat shield attached to one of said longitudinal support elements and extending longitudinally of -said bulb and being substantially transverse to the plane of said filament, said heat shield comprising: a metallic foil member, having a plurality of openings, mounted on o-ne of said support elements at the side of said filament whereby when said lamp is operated with its axis horizontal and said shield above the filament, said shield will be in the path of convection currents bearing heat emanating from said filament said apertures forming passages for a portion of said currents and said member obstructing the remainder of said currents whereby heat from the filament is distributed over a greater portion of the envelopes surface.

4. The shield defined in claim 3 in which the shield is bent along a longitudinal center line to form two panels each obliquely oriented relative to the plane of said filament.

5. In a projector, an incandescent lamp having a gasfilled, tubular envelope, an upper and a lower support element extending laterally within said envelope, and a filament mounted in a vertical plane between said elements, .a heat shield transverse to the plane of said filament and comprising: a screen of metallic wires, having a plurality of openings, mounted on said upper element above said filament and in the path of convection currents bearing heat emanating from said filament said openings forming passages for a portion of said currents and said screen deflecting the remainder of said currents whereby heat from the filament is distributed vover a greater portion of the envelopes surface.

6. The lamp 0f claim 1, in which the heat shield is transverse to the plane of the filament.

References Cited in the filefof this patent UNITED STATES PATENTS ,1,903,144 Spanner Mar. 28, '1933 1,924,319 Hull Aug. 29, .1933 1,967,906 VKRuttenaueret'al Iuiy'24, 1934 

